windows: seek to the end of posixfile when opening in append mode
The position is implementation defined when opening in append mode,
and it seems like Linux sets it to EOF while Windows keeps it at zero.
This has caused problems in the past when a file is opened and tell()
is immediately called, such as 48c232873a54 and 6bf93440a717.
Since the only caller of osutil.posixfile is this windows module, this seems
like a better place to fix the issue than in osutil.c and pure.osutil.
Testing that hghave does not crash when checking features
$ "$TESTDIR/hghave" --test-features 2>/dev/null